I agree that evaluation is the only way to go, because it's the only way you'll know how the product will operate in your environment. I disagree, however, that it will cost a lot of time and money if you are evaluating software. You can download VMWare Server for Windows or Linux from VMWare for free. Create virtual testing labs using distros you also can download for free. Microsoft are also now very generous with their evaluation products, with most durations between 60 and 180 days. All it takes is your time, but again, once you have a basic Windows 2003 Server image, take a copy or snapshot of it, and clone other virtual machines from it rather than sitting through the install every time (just remember to use something like NewSID to change the SID each time, or you can get really weird errors when machines with the same SID are members of the same domain).

A lot of hardware/appliance vendors will also do a free eval, direct or via a channel partner, if you seem like a legitimate opportunity. You'll have to be prepared to deal with the sales people, though, because once a company has done an on-site eval they really, really want to make the sale.